Aircraft range capabilities have increased over the decades, allowing flight operations to increasingly traverse remote areas where the aircraft (or airplane) is at times far from the nearest airport. The International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPs) set rules for two-engine (twin-engine) commercial air travel. The rules permit twin-engine commercial air travel on flight paths (routes) that extend beyond a distance of 60 minutes of flying time at one-engine inoperative speed from a nearest airport if the aircraft and operator are approved for such operations. These operations are referred to as extended operations, or under the acronym ETOPS (Extended Operations).
First flown in 1985, ETOPS is a conservative, evolutionary program that enhances safety, reliability and efficiency during extended operations. An enormously successful part of many operators' daily operations, ETOPS benefits passengers and operators (airlines) alike. To passengers, it means greater choice in flights, greater convenience in departure and arrival times, and a higher likelihood that flights will be direct and on-time. To operators (airlines), ETOPS means greater profitability and flexibility, because it makes the superior efficiency and reliability of two-engine aircraft available on routes that were once the exclusive domain of three- and four-engine aircraft.
Although ETOPS stands for extended operations, it may be more accurately described by the alternative phrase “extended-diversion-time operations” (EDTO). ETOPS or EDTO flights are those conducted on routes that at some point take the aircraft far from an airport, and a long diversion may be required were the flight crew to elect to divert and fly to an alternate airport.
The ETOPS program uses a proven “preclude” and “protect” philosophy to mitigate associated risks. This dual philosophy seeks to “preclude” diversions through design and maintenance requirements that enhance the reliability and robustness of aircraft, and their engines and systems. Diversions can never be entirely eliminated, however, since most result from passenger illness, weather and other factors unrelated to the functioning of the aircraft and its systems. And because diversions can never be entirely eliminated, ETOPS seeks to “protect” those diversions that do occur through robust aircraft and system design as well as operational requirements such as airport planning, a fuel reserve, a passenger-recovery plan and so on.
Many ETOPS requirements are based on risk analysis or estimation. Among its more notable design requirements, ETOPS requires increased engine (propulsion) reliability to reduce engine-related risks. These risks are often characterized by the risk of losing thrust from either or both engines during flight of a two-engine aircraft, with the in-flight failure of an engine oftentimes being characterized as an in-flight shutdown (IFSD).
For a two-engine aircraft on an ETOPS flight, one risk of prominent concern is the loss of thrust from both engines (dual-engine failure or shutdown). The necessary failure scenario is a sequence that begins with a first engine shutdown (IFSD) during an ETOPS flight, followed by a non-restartable second engine IFSD before touchdown at the destination airport or an alternate airport, followed by an inability of the first engine to restart. This scenario may be referred to as a two (or dual) independent engine failure scenario, and its sequence may be referred to as a two (or dual) independent engine shutdown sequence.
ETOPS regulations generally require low IFSD rates, and these rates are underpinned by a number of different (loss of thrust) risk models (or equations) that have been developed. These risk models may only roughly approximate risk, and in particular, risk of dual independent engine shutdown. Most of the current models generally have similar shortfalls. The current models generally treat an ETOPS flight as a whole and try to average the risk over the whole flight. To do this, the current models make assumptions and generalizations, usually leaning toward conservative choices. A summed risk of the conservative choices may overwhelm an actual risk, and the summed risk may portray a risk that is substantially higher than an actual risk. Another problem with use of the current models is lack of versatility and visibility. There is substantially no way to adjust the current models based on varying operations or assumptions, or to examine how estimated risk builds as a flight progresses.
Therefore, it may be desirable to have a system and method that improves upon existing practices.